1. Field of the Invention
The present invention relates to a backlight system used as an illumination means for a thin display such as a liquid crystal display (LCD) or the like.
2. Description of Related Art
Generally, a flat panel display such as an LCD panel used in a mobile phone, a notebook computer or the like includes a flat panel and a backlight system combined with the flat panel to illuminate it. An important condition as the backlight system is to provide a uniform light distribution having a sufficient strength throughout the entire surface of the flat panel in the LCD panel and so on.
Conventionally, there is known a backlight system using a light-guiding plate and a prismatic sheet, as shown in FIGS. 8A to 8C (for reference, see JPA 2003-59321, FIGS. 1 and 5).
X, Y, and Z directions in orthogonal coordinates, as shown in FIGS. 8A to 8C show a width direction, a thickness direction and a length direction of the backlight system 110, respectively. In FIGS. 8A to 8C, reference numeral 101 denotes a light-guiding plate, 102 an LED as a light-emitting source disposed at a position facing a light-entrance surface 100c which corresponds to a side surface of the light-guiding surface 101, 103 a prismatic sheet disposed to ice an upper surface 101a of the light-guiding plate 101, and 104 a reflecting sheet disposed to face a lower surface 101b of the light-guiding plate 101.
The upper surface 101a of the light-guiding plate 101 is formed in a smooth see, and the lower surface 101b of the light-guiding plate 101 is provided with a plurality of prisms 101p having prismatic surfaces standing obliquely to the Z direction at a relatively small inked angle (see α, in FIG. 9). On the other hand, a lower surface 103b of the prismatic sheet 103 facing the light-guiding plate 101 is provided with a plurality of prisms 103p projecting at acute angles, for example, 60 to 75 degrees.
In the backlight system 110, as shown in FIG. 9, a beam S1 from the LED 102 entering the light-entrance surface 101c of the light-guiding plate 101 at an angle θ is refracted by the light-entrance surface 101c and enters surfaces of the prisms 101p on the lower surface of the light-guiding plate 101 at an angle lesser than the angle θ by refraction of the light-guiding plate 101.
Here, the beam S1 comprises a flux of parallel light. At this time, if an incident angle to the surfaces of the prisms 101p is more than a critical angle for example, it is 40 degrees, if a refraction index of the light-guiding plate 1 is 1.55), the beam S1 is reflected on the surfaces of the prisms and enters the upper surface 101a of the light-guiding plate 101. Here, if an incident angle to the upper ace 110a is lesser than a critical angle, the beam passes through the upper surface 110a by refraction of the light-guiding plate 101 and emits to the outside. However, if the incident angle is the critical angle or more, the beam is reflected on the upper sure and enters the sues of the prisms 101p on the lower surface 101b, as shown in FIG. 9. Because the prisms 101p have inclined angles α, the incident angle of the entered beam S1 in the upper surface 101p as the reflected light decreases by 2α every time that the beam is reflected on the prisms 101p. Therefore, after the beam is reflected on the surfaces of the prisms 101p one time or multiple times, the incident angle to the upper surface 101a is lesser than the critical angle, the beam passes through the upper surface 101a by refraction thereof and emits to the outside as a beam S12 having an exit angle φ.
Here, the inclined angle α is often set to a small angle in order to improve the transfusion of inside light in the light-guiding plate 101. In this case, even though the incident angle to the upper surface 101a is lesser than the critical angle, the incident angle becomes an angle close to the critical angle. Accordingly, the exit angle φ is easy to be a large angle, for example, an angle exceeding 60°.
FIGS. 10A and 10B illustrate the directivity of the emitted light from the upper surface 101a of the light-guiding plate 101, as shown in FIG. 9.
FIG. 10A illustrates the directivity in a ZY plane, and FIG. 10B illustrates the directivity in an XY plane.
Referring generally to FIG. 10A, the emitted light from the upper surface shifts to the Z direction, the emitted light has less components in the Y direction or a perpendicular direction. In such a state, it is not possible to provide sufficiently a component of illumination light in the perpendicular direction effective to illuminate an object, for example, an LCD panel or the like and thus acquiring brightness for illumination required to the backlight system is difficult. Therefore, the prismatic sheet 103 is disposed in order to direct the emitted light from the light-guiding plate 101 to the Y direction.
Meanwhile, there is a case that the incident angle of the inside light S1 entering the surfaces of the prisms 101p on the lower surface of the light-guiding plate 101, directly or through the reflection on the upper surface 101a is lesser than the critical angle. In this case, the entered light in the light-guiding plate passes through the surfaces of the prisms 101p by the refraction thereof, arrives the reflecting sheet 104 (see FIG. 5A), is reflected thereon, passes through the surfaces of the prisms 101p, again, and enters the light-guiding plate 101. In this way, it is possible to increase usability of the inside light in the light-guiding plate.
FIG. 11 illustrates an operation of the prismatic sheet 103. As viewing from the ZY plane as shown in FIG. 11, the emitted beam S12 from the upper surface 101a of the light-guiding plate 101 enters an inclined surface 103pa of each of the prisms 103p provided on the lower surface of the prismatic sheet 103 facing the light-guiding plate 101, at an angle close to perpendicular, in other words, an incident angle close to 0 degree, goes in the prismatic sheet 103 without refracting almost, enters an inclined surface 103pb opposite across ridge lines of each prism, is reflected totally thereon, goes approximately perpendicularly upward or the Y direction, and emits from the upper surface 103a of the prismatic sheet 103 without refracting almost as an illumination beam 13. Here, the upper surface 103a of the prismatic sheet 103, which corresponds to a final exit surface is formed in a smooth surface or simple rough surface. In this way, because the beam S12 of the emitted light from the light-guiding plate is reflected on the prismatic sheet 103 directly almost as a flux of parallel light and alternates the direction only, the illumination beam S13 as the backlight system which is a final exit light is substantially a parallel beam, the directivity of the illumination light collecting the parallel beam is narrow in the length direction of the backlight system or the Z direction. Even if the final exit surface is the simple rough surface, the directivity in the Z direction has a slight broadening.
FIG. 12A illustrates the directivity of light in the ZY plane, and FIG. 12B illustrates the directivity of light in the XY plane. As is clear from the drawings, the directivity of the backlight system has a large component with respect to the Y direction, an amount of light entering a flat panel (not shown) is increased efficiently, hence enhancing the entire brightness for illumination is effective. However, a width of directivity in the Z direction as shown in FIG. 12A is less considerably in comparison with that of the X direction as shown in FIG. 12B to obstruct a uniform illumination. Therefore, there is a possibility of generating a stripe pattern of bright and dark contrasting. As a result, there is a problem that quality of illumination is lower.